User terminal antenna arrangement for multiple-input multiple-output communications

ABSTRACT

Multiple input multiple output (MIMO) systems are able to provide wireless communications at increased data rates. These systems involve using a plurality of antennas on both the transmitter and receiver and the problem of how to accommodate such structures in user terminals, which are often required to be hand held then arises. An antenna arrangement which is suitable for MIMO communications and which can be incorporated into a user terminal such as a personal digital assistant (PDA) is described. A pair of co-polarised umbrella monopoles and a slot antenna are incorporated into a flap which extends at approximately 90° to the body of the PDA. A fourth monopole is mounted and protrudes from the PDA itself.

FIELD OF THE INVENTION

The present invention relates to an antenna arrangement suitable foruser terminal multiple-input multiple-output radio communicationssystems.

BACKGROUND TO THE INVENTION

The demand for wireless communication systems has grown steadily overrecent decades, and has included several technological jumps over thistime, particularly in the area of cellular and wireless local areanetwork (WLAN) communication systems. Analogue cellular phones have beenreplaced with digital handsets using for example GSM and CDMAtechnologies, and so called third generation systems such as UMTS arenow being introduced. Similarly WLAN technologies such as HyperLan andIEEE 802.11b are also being introduced. The number of users continues toincrease and data traffic is now becoming an important part of thewireless network. Both of these factors mean that it is important foroperators to look for methods of increasing the capacity of theirnetworks to meet future demands.

An increasing demand to access the internet and other data networks hasled to the need for increased capacity particularly in the downlinkdirection. That is, increased capacity from basestations to wirelessuser terminals in order that users are able to download web pages andother information. One way of addressing this problem is to use multipleinput multiple output (MIMO) systems which are able to provide increaseddata rates. These systems involve using a plurality of antennas on boththe transmitter and receiver and the problem of how to accommodate suchstructures in user terminals, which are often required to be hand heldthen arises.

In our earlier U.S. patent application Ser. No. 09/765,532, filed on 19Jan. 2001 and also assigned to Nortel Networks, we describe an antennaarrangement for use in a laptop computer to provide MIMO communications.Three or four antennas are provided around the edges of the screen partof the laptop computer which is preferably oriented vertically when inuse. The antennas are all integrated into the body of the laptop wherethe body includes the screen part. Transmission line loaded antennas areused such as bent folded monopoles or planar inverted F antennas. Eachantenna has a monopole section which is perpendicular to a ground plane.Whilst the systems described in Ser. No. 09/765,532 are fully workablethe present invention seeks to extend our earlier work.

Considering hand-held user terminals such as personal digital assistants(PDAS) these are currently available from a range of manufacturers suchas Palm, Handspring and IBM. Some PDAs such as the Palm VIIx areprovided with wireless communications capability, either using atransceiver built into the PDA, or by mounting the PDA in a separatecradle. However, these systems provide a single channel wirelesssolution in the same way as conventional mobile terminals rather thanenabling MIMO communications.

The invention seeks to provide an antenna arrangement suitable for userterminal multiple-input multiple-output radio communications systemswhich overcomes or at least mitigates one or more of the problemsmentioned above.

Further benefits and advantages of the invention will become apparentfrom a consideration of the following detailed description given withreference to the accompanying drawings, which specify and show preferredembodiments of the invention.

SUMMARY OF THE INVENTION

According to an aspect of the present invention there is provided anantenna arrangement suitable for use in a user terminal in order toprovide wireless multiple-input-multiple-output (MIMO) communications,said antenna arrangement comprising:

-   -   at least a first and a second ground plane, the first ground        plane arranged to be provided in a body of the user terminal and        the second ground plane arranged to be provided external to the        body of the user terminal; and    -   two or more antennas each associated with one of the ground        planes, at least one of the antennas being associated with the        first ground plane and at least another of the antennas being        associated with the second ground plane.

For example, the antenna arrangement can be used in a user terminal suchas a PDA, a mobile telephone or any other suitable type of userterminal. By using two or more antennas MIMO communications can beachieved. Also, by having one ground plane external to the user terminalbody problems as a result of space restrictions in the user terminalbody are avoided, whilst at the same time enabling the plurality ofantennas to be accommodated. For example, the user terminal may be aPDA, laptop computer or other device. The electronic components whichprovide the functionality for the PDA or other device are provided inthe body of the user terminal. By providing one ground plane for theantenna arrangement external to the body it is possible to provide aMIMO capable antenna arrangement without needing to alter the existingelectronics in the body of the PDA. That is, the antenna arrangement isaccommodated using the user terminal body and external ground planewithout affecting components within the user terminal body that providefunctionality independent of the antenna arrangement.

Preferably, the antennas are arranged and positioned in order thatangular coverage provided by the antennas in use is maximised in atleast one plane. For example, the antennas can be arranged to besubstantially omnidirectional in an azimuth plane in use. This providesthe advantage that communications can be established no matter whichparticular orientation the terminal is given.

Preferably, at least some of said antennas are arranged to providespatial diversity. This provides the advantage the effects of spatialfading can be avoided as far as possible.

In a preferred embodiment the ground planes are positioned with respectto one another in use such that polarisation diversity is providedbetween antennas associated with different ones of the ground planes.Also, polarisation diversity for any orientation of the antennaarrangement can be achieved,

Preferably, the user terminal (including the antenna arrangement) isarranged to be hand held. This provides the advantage that the antennaarrangement can enable MIMO communications to a terminal such as a PDAor mobile telephone and give increased data rates to that terminal.

Advantageously, the user terminal comprises a display and one of theground planes is substantially parallel with the display. For example,the user terminal can be a PDA with a display face. Circuit boardswithin the PDA itself, which provide the functionality of the PDA,effectively form a ground plane which is substantially parallel to thedisplay face. By making use of this ground plane an antenna arrangementthat can be incorporated into the PDA structure can be achieved, despitethe size restrictions imposed by the PDA structure.

In a preferred embodiment another of said ground planes is arranged tobe provided in a flap moveably connected to the user terminal. Thisprovides the advantage that at least part of an antenna arrangement canbe incorporated into the flap where more space is available than in thePDA body itself. For example, the flap is of a similar area to thedisplay face of the PDA. Because the flap is connected to the userterminal the antenna arrangement in the flap can also easily bepermanently connected to the user terminal. This is advantageouscompared to systems using cradles or other separate devices which needto be connected to a terminal and which can easily be lost or damaged.Also, because the flap is moveably connected to the user terminal, forexample, using a hinge, it is easy to operate. The PDA can be supportedon a table in use for example, by being propped up using the flap.Alternatively, the PDA can be held in the hand with the flap extendingover and behind the user's hand.

Preferably, the antenna arrangement comprises a first pair of antennasand a second pair of antennas, said first pairs of antennas beingpolarisation diverse with respect to the second pair of antennas. Usingthis type of system four antennas are provided in two pairs which havepolarisation diversity and can advantageously be used in a MIMOcommunications system.

Preferably, the first pair of antennas is fed from a first ground planeand only one of the second pair of antennas is also fed from that firstground plane. For example, the first ground plane can be incorporatedinto the flap of a PDA as mentioned above. This provides the advantagethat three of the four antennas are associated with a ground plane thatis not in the body of the PDA itself, where space restrictions are harshand many conductors are present. However, it is also possible toincorporate two or three of the antennas in the body of the PDA itselfas explained in more detail below.

Advantageously, the first pair of antennas are spaced as far apart fromone another as possible whilst still being suitable for accommodation inthe user terminal. For example, these two antennas are provided in theflap of a PDA as mentioned above. By spacing those antennas as far apartfrom one another as possible spatial diversity is provided to alleviateproblems associated with spatial fading.

Preferably, said first pair of antennas are selected from monopoles andumbrella monopoles. This provides the advantage that the antennas are ofa simple structure which is easy to install. Also, the antennas providegood omnidirectional coverage in an azimuth plane. Umbrella monopolesare advantageous in situations where space is restricted. These are atype of top-loaded monopole. Top-loaded monopoles are preferred becausethese have a 2D structure which can be used to achieve a slim-lineantenna structure that is suitable for PDAs. However, normal mode spiralstructures can also be used provided sufficient depth is available forsuch 3D structures.

In a preferred embodiment one of the second pair of antennas is a slot.This provides the advantage that a simple antenna structure that is easyto install is provided. Also the radiation pattern characteristics aresimilar to that for the monopole structure mentioned above but with theopposite polarisation. The slot can be shortened to save space, forexample, by forming a dumbbell.

Preferably the other of the second pair of antennas is a monopole and isarranged to protrude from the user terminal in use. This provides theadvantage that good omnidirectional coverage in an azimuth plane isachieved. Also the antenna structure is simple and can easily bearranged to fold away against the body of the user terminal or toretract into the body of the user terminal when not in use.

In another embodiment the antenna arrangement, when used as part of acommunications device, comprises fewer receive chains than there areantennas. This provides the advantage that costs are reduced becausereceive chains are relatively expensive. Also, the advantage that it ispossible to select those antennas which provide the best performance atany one time is achieved. For example, the antennas can be arranged tohave directional antenna patterns that differ from one another and whichare designed to take into account blocking caused by the user terminalstructure itself. This is also advantageous in the situation that aplurality of such user terminals are being used simultaneously. By usingdirectional antennas, interference levels are reduced and carrier tointerference levels improved when considering all the user terminalstogether. In order to select which antennas to use a switching means canbe provided to allow the receive chains to be switched between theantennas. Alternatively, adaptive combination of the antennas is used asdescribed in more detail below.

Preferably, each antenna is substantially co-planar with its associatedground plane. This provides the advantage that a slim-line design isachieved which is suitable for use in user terminals whose depth islimited.

In another embodiment, each of the antennas is directional in that ithas a directive gain in at least one of two different directions. Thisprovides the advantage that carrier to interference levels are improvedas compared with situations in which all the antennas areomnidirectional.

According to another aspect of the present invention there is provided auser terminal comprising an antenna arrangement suitable for providingwireless multiple-input-multiple-output (MIMO) communications, saidantenna arrangement comprising:

-   -   at least a first and a second ground plane, the first ground        plane being provided in a body of the user terminal and the        second ground plane being provided external to the body of the        user terminal; and    -   two or more antennas each associated with one of the ground        planes, at least one of the antennas being associated with the        first ground plane and at least another of the antennas being        associated with the second ground plane.

The invention also encompasses a multiple-input-multiple-outputcommunications system comprising a user terminal as defined immediatelyabove and a signal processor arranged to implement a space time codingcommunications transmission or reception method. The signal processor isincorporated in either the PDA body and/or the flap.

This provides the advantage that the data rate of communications withthe user terminal are improved as compared with a multi-beam antennasystem for example.

According to another aspect of the present invention there is provided amethod of transmitting or receiving space time coding communicationsusing a user terminal comprising the steps of:

-   -   providing at least a first and a second ground plane, the first        ground plane being provided in a body of the user terminal and        the second ground plane being provided external to the body of        the user terminal; and    -   providing two or more antennas each associated with one of the        ground planes, at least one of the antennas being associated        with the first ground plane and at least another of the antennas        being associated with the second ground plane.

The invention is also directed to a method by which the describedapparatus operates and including method steps for carrying out everyfunction of the apparatus.

The preferred features may be combined as appropriate, as would beapparent to a skilled person, and may be combined with any of theaspects of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to show how the invention may be carried into effect,embodiments of the invention are now described below by way of exampleonly and with reference to the accompanying figures in which:

FIG. 1 is a schematic diagram of a prior art MIMO wirelesscommunications system;

FIG. 2 is a side view of a personal digital assistant (PDA)incorporating an antenna arrangement;

FIG. 3 is a perspective view of an antenna arrangement for use in a PDA;

FIG. 4 is a schematic diagram of the antenna arrangement of FIG. 3;

FIG. 5 is a graph showing the return loss for the antennas in thearrangement of FIG. 3;

FIG. 6 is a graph showing mutual coupling between the antennas in thearrangement of FIG. 3;

FIG. 7 comprises graphs of the azimuth radiation patterns of theantennas of the arrangement of FIG. 3;

FIG. 8 is a graph showing the radiation patterns of the antennas of thearrangement of FIG. 3;

FIG. 9 is a graph of optimised complementary radiation patterns;

FIG. 10 is a graph of radiation patterns for directional antennas.

DETAILED DESCRIPTION OF INVENTION

Embodiments of the present invention are described below by way ofexample only. These examples represent the best ways of putting theinvention into practice that are currently known to the Applicantalthough they are not the only ways in which this could be achieved.

The term “directional antenna” is used to refer to an antenna which hasa pattern that is not omnidirectional and which has significantdirective gain in a particular direction.

A MIMO wireless communication system (see FIG. 1) is one which comprisesa plurality of antennas 10 at the transmitter 11 and a plurality ofantennas 12 at the receiver 13. The antennas 10, 12 are employed in amulti-path rich environment such that due to the presence of variousscattering objects (buildings, cars, hills, etc.) in the environment,each signal experiences multipath propagation. Thus a cloud shape 14 isshown in FIG. 1 to represent the scattered signals between the transmitand receive antennas. User data is transmitted from the transmitantennas using a space-time coding (STC) transmission method as is knownin the art. The receive antennas 12 capture the transmitted signals anda signal processing technique is then applied as known in the art, toseparate the transmitted signals and recover the user data.

MIMO wireless communication systems are advantageous in that they enablethe capacity of the wireless link between the transmitter and receiverto be improved compared with previous systems in the respect that higherdata rates can be obtained. The multipath rich environment enablesmultiple orthogonal channels to be generated between the transmitter andreceiver. Data for a single user can then be transmitted over the air inparallel over those channels, simultaneously and using the samebandwidth. Consequently, higher spectral efficiencies are achieved thanwith non-MIMO systems.

One problem with existing MIMO systems concerns the large size of thetransmit and receive antenna arrays. Previously, MIMO transmit andreceive antenna arrays have used spatially diverse antenna arrays. Thatis, the spacing between the individual antenna elements is arranged tobe large enough such that decorrelated spatial fading is obtained. Thisis necessary in order to prevent the number of orthogonal channels frombeing reduced. That is, if the fading characteristics between antennaelements is similar (correlated) then the number of orthogonal channelsthat can be realised is reduced. However, as described in our earlierU.S. patent application Ser. No. 09/975,653 it is possible to usepolarisation diverse antenna elements that are positioned closely toproduce a compact antenna arrangement whilst still enabling MIMOcommunications to be provided.

The present invention recognises that for MIMO antenna systems inparticular, it will be desirable to provide two or more diverse antennaelements in a compact radio terminal. In order to take full advantage ofthe propagation channels available it is important that the antennasystem can receive signals of two orthogonal polarisations and have asfull an angular coverage in the azimuth plane as possible. Good angularcoverage in the azimuth plane is desired because the antenna arrangementis preferably incorporated into a portable user terminal. The particularorientation of the user terminal will then vary depending on thatpreferred by the user in a particular environment. Thus a signalreceived from a basestation at the user terminal could arrive at anyprinciple angle relative to the orientation of the user terminal. Inpractice there will be angle spread about the principal direction, forexample of the order of 45° for an outdoor to indoor path and this isadvantageously exploited by a MIMO system to provide parallel channelsfrom reasonably closely spaced antenna elements, as described in detailbelow. As well as the requirements mentioned above, it is desired toprovide as much spatial diversity as possible between two antennas oflike polarisation in order to reduce the effects of spatial fading. Thisis less important between antennas of unlike polarisation because thepropagation channel provides decorrelation.

A preferred embodiment of the present invention is now described indetail. This embodiment concerns an antenna arrangement for a PDAalthough the arrangement is also applicable to other types of terminal.Also, whilst the antenna arrangement has particular application in MIMOcellular and WLAN application, the antennas could also be used in otherapplications such as standard receive or transmit diversity for example.

As shown in FIG. 2 a PDA 20 is provided with a support 21 which can beused to prop up the PDA in use as shown. In a particularly preferredexample, the angle 22 between the PDA and the support 21 is about 90°,although this is not essential as described in more detail below. Forexample, the support 21 can be a flap that is moveably connected to thePDA such that it covers a display screen face of the PDA when not inuse. The flap can be arranged to fold around the side or over the top ofthe PDA. Alternatively the support 21 can be separate to the PDA, or maybe a cover or case that is removable and can be used as a support asshown in FIG. 2. The support is arranged to accommodate at least oneground plane for an antenna arrangement as well as one or more antennas.

In a preferred embodiment the flap is hinged to a side of the PDA andcovers a display screen face of the PDA when the PDA is not in use. Inuse, the flap is extended behind and used to prop up the PDA as shown inFIG. 2. Antennas of the antenna arrangement are then integrated intoeither the flap or the PDA itself. Preferably the antennas areintegrated into the flap (or cover or other support), in which morespace is available than in the PDA. The PDA (or other type of userterminal) itself contains electronics for providing the functionality ofthe PDA and it is required not to disrupt or alter these. In addition,interference to the antenna operation caused by the electronics in thePDA body needs to be avoided. By using the flap (or other structure)which is external to the PDA body these problems are addressed.

Preferably four antennas are used, three incorporated into the flap andone mounted on the PDA itself. However, this is not essential. Anysuitable number of antennas can be used. FIG. 3 shows a preferredembodiment of an antenna arrangement with three antennas 31, 32, 33associated with ground plane 35 and one antenna 34 associated with asecond ground plane 36. The first ground plane 35 and its threeassociated antennas is arranged to be incorporated into the flap 21 ofFIG. 2 and the second ground plane is arranged to be incorporated intothe PDA itself. The ground planes may be of metal or metallized plastic.For example, the PDA body contains circuit boards for providing thefunctionality of the PDA itself. These circuit boards effectivelyprovide the first ground plane 35. The antenna 34 that is associatedwith the second ground plane 36 is preferably mounted on the PDA so thatit protrudes from the top of the PDA as shown in FIG. 3. It can beretractable or arranged to fold against the body of the PDA when not inuse.

The three antennas to be provided in the flap are illustrated in FIG. 4in more detail. The dimensions shown in FIG. 4 are those used in aparticular arrangement from which empirical measurements were taken asdescribed below. However, these dimensions are examples only and othersuitable sized components can be used. Preferably the four antennas31-34 comprise two pairs of antennas. In this particular example,antennas 31 and 33 are co-polarised at +45° when viewed from aparticular orientation and antennas 32 and 34 are co-polarised at −45°when viewed from a particular orientation. Within each pair of antennas,those antennas are spaced as far apart as possible in order to providespatial diversity. Thus antennas 31 and 33 are mounted at oppositecorners of the flap (in this case, bottom left and top right of theflap). However, it is not essential to mount antennas 31 and 33 atopposite corners of the flap; any suitable separation between thoseantennas can be used depending on the amount of spatial diversityrequired.

When the flap is positioned at 90° to the PDA body then identicalantennas mounted on the flap and PDA will be oppositely polarised. Thisis relevant for the other pair of antennas, 32, 34 where one antenna 32is mounted on the flap and one 34 on the PDA body. However, it is notessential for the flap to be positioned at 90° to the PDA body. The aimis to create polarisation diversity between antennas associated with thetwo ground planes. One ground plane is provided in the PDA body and onein the flap. These two structures are arranged to be separated in usesuch that polarisation diversity is provided for any orientation of theterminal in an azimuth plane with respect to another entity with whichcommunications are to be effected (e.g. a basestation). For example, inthe case of the PDA with a flap, the separation between the two groundplanes may only be about 60°.

Any suitable types of antenna can be used taking into account the spaceavailable, radiation pattern requirements, installation requirements,and polarisation requirements. For example, monopole antennas, dipoleantennas, planar inverted F antennas, patch antennas or slot antennas.

In a preferred embodiment as illustrated in FIG. 4, a first pair ofantennas 31, 34 are monopoles and these are mounted on the flap.

Monopoles provide good azimuth coverage and are easy to install. Inorder to reduce the height required for the monopoles these arepreferably top loaded as illustrated in FIG. 4. If top loading does notreduce the length of the monopoles enough, the loading is placed closeenough to the monopole to couple to it as illustrated in FIG. 4. Thus ina preferred example, the monopoles are umbrella monopoles with a centralstrip and two side strips which fan out from the base of the centralstrip at about 20° to each side as known in the art. The umbrellamonopoles are printed within the flap area, one facing up and one facingdown and in the particular example of FIG. 4 are formed from coppermounted on a board. Umbrella monopoles are described in more detail in“Small Antennas” by K. Fujimoto, A. Henderson, K. Hirasawa and J. F.James (Research Study Press 1987).

For the second pair of antennas, one antenna 32 is preferably providedin the form of a slot 32 mounted in the flap. Preferably the slot 32 isshortened by top loading in a manner analogous to top loading ofdipoles. Slot antennas have similar size and radiation patterncharacteristics as dipole antennas and have opposite polarisation. Inthe particular example of FIG. 4 the ground plane 35 is formed fromcopper. The other antenna of the second pair is mounted on the PDA bodyand preferably comprises a monopole.

In this way as many different antenna orientations as possible are used,with dual polarisation and some space and pattern diversity. The azimuthcoverage of all four antennas is reasonably uniform, thus providing fourdiverse signals with comparable signal strength, as required for MIMO ordiversity antennas.

The match of the umbrella monopoles 31, 33 is sensitive to the proximityof the edge of the ground plane 35 and we identified their optimumposition empirically. This optimum position for the arrangement of FIG.4 is identified by the dimensions given in FIG. 4. As described abovethe separation of the umbrella monopoles 31, 33 was maximised to providespatial diversity to give the positions shown in FIG. 4. It was foundthat further increase in separation of those umbrella monopoles degradesthe match to worse than −10 dB. FIG. 5 shows the return loss of antennas31 to 34 measured in situ on the PDA (or flap). Results for antenna 31are shown by line 31 in FIG. 5 and so on for the other antennas. FIG. 5shows that all four of the antennas is operable within the PCS 1900frequency band. That is, each of the four antennas has a return loss ofat least 10 dB between 1850 MHz and 1990 MHz.

For the antennas to behave independently and to have phase centresseparated as widely as possible it is important that the elements arenot too tightly coupled to one another. This was assessed by directlymeasuring the coupling between the elements and the results are shown inFIG. 6. There are six possible pairs of antennas that can be formed fromthe four antennas of FIG. 3, namely (31, 32), (31, 33), (31, 34), (32,33), (32, 34), (33, 34), and the six lines in FIG. 6 show the couplingfound between each of those pairs. Although some coupling was found asshown in FIG. 6 this was within reasonable levels such that a workablesystem results.

FIG. 7 shows the azimuth plane radiation patterns for antennas 31 to 34.The antennas were mounted on the PDA (or flap) and had nominalpolarisation of +/−45° as described above with reference to FIG. 4. Inall cases the patterns show variation of the gain and polarisation as afunction of angle. This is largely due to scattering from the PDA. It isalso apparent that antennas 31, 32 and 33, which are mounted on the flaphave relatively low gain in the boresight direction. This is due toblocking by the body of the PDA. Antenna 34 is unaffected by thisbecause it protrudes above the PDA. FIG. 8 shows the same data as FIG. 7but plotted as total radiated power, with the four antennas superposedon the same plot. The lower gain of antennas 31 to 33 in the boresightdirection is apparent.

The results shown in FIGS. 5 to 8 indicate that the antennas of thearrangement of FIG. 4 radiate effectively. Some scattering occurs as aresult of the PDA although this does not significantly detriment theoperation of the antenna arrangement. The scattering causes polarisationmixing in the patterns and results in reduced gain in some directions.Also the proximity of the elements to one another results in somecorrelation of the radiation patterns. However, none of these factorswere found to significantly detriment the operation of the antennaarrangement and the benefits of enabling MIMO operation to be providedin a compact antenna arrangement are achieved.

In another embodiment, fewer receive chains than antennas are providedfor use in the user terminal. For example, consider the arrangement ofFIG. 4. In that case, four antennas are provided and in one embodimenttwo of those four antennas are selected for use at any one particulartime and situation. This provides the advantage that only two receivechains are required which reduces costs and space requirements. Also, itis not necessary for all the antennas to have omnidirectional patterns.Instead antennas with directional patterns can be selected in order toprovide good coverage when considered together rather than individually.

An advantage of using antennas with omnidirectional patterns is that theunknown orientation of the user terminal when in use can be accountedfor and four equal paths to the receiver provided. These four pathsenable capacity to be increased by using a MIMO system if the paths aredistinguishable. However, as shown in FIG. 8, the umbrella monopoles(31, 33) exhibit dips in the centre of the radiation pattern as does thefolded slot 32. This can be attributed to blocking by the body of thePDA as mentioned above. The external monopole pattern 34 exhibits a muchmore omnidirectional pattern and can be seen to be more consistentacross the azimuth plane.

In the embodiment where two from four antennas are selected the antennapatterns are advantageously complementary. That is, across the azimuthplane two of the four antennas are required at each angle to provideadequate signal strength for each uncorrelated path. In the embodimentdescribed above with reference to FIG. 4 the antenna patterns weredesigned to be omnidirectional, but the profile of the terminal limitedthe amount of omnidirectionality achieved due to blocking.

In using complementary directive patterns we can not only avoid this butalso design the patterns with the body of the PDA in mind. For example,the four antennas of FIG. 4 are preferably arranged to provide theantenna patterns of FIG. 9. Considering traces in FIG. 9 it can be seenthat at any angle, two antenna patterns provide good signal strength.For example complementary patterns 91 and 92 could initially be selectedand if the signal strength was found to be poor the other patterns 93and 94 selected. Alternatively, both options could be tested and thebest pair of antennas chosen. The selection process may then be repeatedat intervals in situations to take account of changes over time (forexample, movement of the user terminal and of objects around theterminal).

Any suitable mechanism for directing the antennas can be used as knownin the art. For example, the physical arrangement and position of theantennas may provide directionality. Alternatively, mechanical steeringcan be used to physically direct the antennas. Also, beamforming can beused to achieve directionality.

Another advantage of using directional antennas and selecting which ofthese to use according to the particular circumstances is that carrierto interference level ratios can be improved when considering aplurality of user terminals as compared with the situation in whichomnidirectional antennas are used. Consider the situation in whichomnidirectional patterns are used. In the angle of the best server thepattern provides us with our required carrier in the required region. Atall other angles however, the omnidirectional pattern provides a highlevel of interference to all other carriers. By using directionalantennas instead, we can improve this performance by selecting a patterndirected towards the best server. At all other angles the directionalpattern is lower in signal strength than before and this reducesinterference. This is illustrated in FIG. 10 which shows antennapatterns 100 for two directional antennas selected in a 2 from 4, 2:2MIMO system and antenna patterns 101 for two omnidirectional antennasalso in a 2:2 MIMO system. We can clearly see from FIG. 10 that in theregion of interest, best C, the signal is much stronger for theselection antennas than for the omnidirectional antennas. In the regionwhere interference is provided to other carriers, the antenna patternsof the directional antennas mean that a much lower signal is seen inthis region by other user terminals, thus improving our overall carrierto interference levels.

1. An antenna arrangement suitable for use in a user terminal in orderto provide wireless multiple-input-multiple-output (MIMO)communications, said antenna arrangement comprising: (i) at least afirst and a second ground plane, the first ground plane (36) arranged tobe provided in a body of the user terminal and the second ground plane(35) arranged to be provided external to the body of the user terminal;and (ii) two or more antennas (31, 32, 33, 34) each associated with oneof the ground planes, at least one of the antennas (34) being associatedwith the first ground plane and at least another of the antennas (31)being associated with the second ground plane.
 2. An antenna arrangementas claimed in claim 1 wherein said antennas are arranged to providepolarisation diversity.
 3. An antenna arrangement as claimed in claim 1wherein said antennas are arranged and positioned in order that angularcoverage provided by the antennas in use is maximised in at least oneplane.
 4. An antenna arrangement as claimed in claim 1 wherein at leastsome of said antennas are arranged to provide spatial diversity.
 5. Anantenna arrangement as claimed in claim 1 wherein said ground planes arepositioned with respect to one another in use such that polarisationdiversity is provided between antennas associated with different ones ofthe ground planes.
 6. An antenna arrangement as claimed in claim 1wherein said user terminal is arranged to be hand held.
 7. An antennaarrangement as claimed in claim 1 wherein said user terminal comprises adisplay and wherein one of said ground planes is arranged to besubstantially parallel with the display.
 8. An antenna arrangement asclaimed in claim 7 wherein another of said ground planes is arranged tobe provided in a flap moveably connected to the user terminal.
 9. Anantenna arrangement as claimed in claim 1 which is arranged to operateat at least one wavelength and wherein the length of the user terminalis about 0.5 to 1 of that wavelength.
 10. An antenna arrangement asclaimed in claim 1 which comprises a first pair of antennas and a secondpair of antennas, said first and pairs of antennas being polarisationdiverse with respect to the second pair of antennas.
 11. An antennaarrangement as claimed in claim 10 wherein the first pair of antennas isfed from a first ground plane and only one of the second pair ofantennas is also fed from that first ground plane.
 12. An antennaarrangement as claimed in claim 10 wherein the first pair of antennasare spaced as far apart from one another as possible whilst still beingsuitable for accommodation in the user terminal.
 13. An antennaarrangement as claimed in claim 10 wherein said first pair of antennasare selected from monopoles and umbrella monopoles.
 14. An antennaarrangement as claimed in claim 11 wherein said one of the second pairof antennas is a slot.
 15. An antenna arrangement as claimed in claim 11wherein the other of the second pair of antennas is a monopole.
 16. Anantenna arrangement as claimed in claim 11 wherein said other of thesecond pair of antennas is arranged to protrude from the user terminalin use.
 17. An antenna arrangement as claimed in claim 1 wherein eachantenna is substantially co-planar with its associated ground plane. 18.An antenna arrangement as claimed in claim 1 wherein each of saidantennas is directional in that it has a directive gain in one of atleast two different directions.
 19. A communications device comprisingan antenna arrangement as claimed in claim 1 and which further comprisesfewer receive chains than there are antennas.
 20. A user terminalcomprising an antenna arrangement suitable for providing wirelessmultiple-input-multiple-output (MIMO) communications, said antennaarrangement comprising: (iii) at least a first and a second groundplane, the first ground plane (36) being provided in a body of the userterminal and the second ground plane (35) being provided external to thebody of the user terminal; and (i) two or more antennas (31, 32, 33, 34)each associated with one of the ground planes, at least one of theantennas (34) being associated with the first ground plane and at leastanother of the antennas (31) being associated with the second groundplane.
 21. A multiple-input-multiple-output communications systemcomprising a user terminal as claimed in claim 20 and a signal processorarranged to implement a space time coding communications transmission orreception method.
 22. A method of transmitting or receiving space timecoding communications using a user terminal comprising the steps of: (i)providing at least a first and a second ground plane, the first groundplane (36) being provided in a body of the user terminal and the secondground plane (35) being provided external to the body of the userterminal; and (ii) providing two or more antennas (31, 32, 33, 34) eachassociated with one of the ground planes, at least one of the antennas(34) being associated with the first ground plane and at least anotherof the antennas (31) being associated with the second ground plane.